1. Field of the Invention
The present invention relates to an active silencer, and more particularly, to an active silencer useful for the prevention of noise in air ducts, intake and exhaust pipes of automobile engines, etc.
2. Description of the Related Art
Active silencers cancel out noise by the provision of sound sources for silencing and by outputting from the sound sources sound having a phase opposite to the noise. Conventional resonance type silencers have been large in size due to the long wavelengths of the low frequency sound, but the above-mentioned active silencers are free from restrictions by the wavelength of the noise and therefore can effectively silence low frequency sound with a compact shape. They have therefore come into use as suitable for automobile silencers where installation space is difficult to secure. These silencers usually use speakers for the above-mentioned sound sources and use microphones for the sound collection means for detection of the noise waveform.
When such an active silencer is provided in a pipe like an intake or exhaust pipe of an automobile engine, where one end is connected to the sound source, i.e., the engine, and the other end is open, there is the problem that even if a microphone is installed in the pipe, it is difficult to accurately detect the waveform of the noise due to the phenomenon of noise reflection at the open end of the pipe.